Strict Standards: Declaration of childClass::customMethod() should be compatible with that of parentClass::customMethod()
What are possible causes of this error in PHP? Where can I find information about what it means to be compatible?
childClass::customMethod() has different arguments, or a different access level (public/private/protected) than parentClass::customMethod().
This message means that there are certain possible method calls which may fail at run-time. Suppose you have
class A { public function foo($a = 1) {;}}
class B extends A { public function foo($a) {;}}
function bar(A $a) {$a->foo();}
The compiler only checks the call $a->foo() against the requirements of A::foo() which requires no parameters. $a may however be an object of class B which requires a parameter and so the call would fail at runtime.
This however can never fail and does not trigger the error
class A { public function foo($a) {;}}
class B extends A { public function foo($a = 1) {;}}
function bar(A $a) {$a->foo();}
So no method may have more required parameters than its parent method.
The same message is also generated when type hints do not match, but in this case PHP is even more restrictive. This gives an error:
class A { public function foo(StdClass $a) {;}}
class B extends A { public function foo($a) {;}}
as does this:
class A { public function foo($a) {;}}
class B extends A { public function foo(StdClass $a) {;}}
That seems more restrictive than it needs to be and I assume is due to internals.
Visibility differences cause a different error, but for the same basic reason. No method can be less visible than its parent method.
if you wanna keep OOP form without turning any error off, you can also:
class A
{
public function foo() {
;
}
}
class B extends A
{
/*instead of :
public function foo($a, $b, $c) {*/
public function foo() {
list($a, $b, $c) = func_get_args();
// ...
}
}
Just to expand on this error in the context of an interface, if you are type hinting your function parameters like so:
interface A
use Bar;
interface A
{
public function foo(Bar $b);
}
Class B
class B implements A
{
public function foo(Bar $b);
}
If you have forgotten to include the use statement on your implementing class (Class B), then you will also get this error even though the method parameters are identical.
I faced this problem while trying to extend an existing class from GitHub. I'm gonna try to explain myself, first writing the class as I though it should be, and then the class as it is now.
What I though
namespace mycompany\CutreApi;
use mycompany\CutreApi\ClassOfVendor;
class CutreApi extends \vendor\AwesomeApi\AwesomeApi
{
public function whatever(): ClassOfVendor
{
return new ClassOfVendor();
}
}
What I've finally done
namespace mycompany\CutreApi;
use \vendor\AwesomeApi\ClassOfVendor;
class CutreApi extends \vendor\AwesomeApi\AwesomeApi
{
public function whatever(): ClassOfVendor
{
return new \mycompany\CutreApi\ClassOfVendor();
}
}
So seems that this errror raises also when you're using a method that return a namespaced class, and you try to return the same class but with other namespace. Fortunately I have found this solution, but I do not fully understand the benefit of this feature in php 7.2, for me it is normal to rewrite existing class methods as you need them, including the redefinition of input parameters and / or even behavior of the method.
One downside of the previous aproach, is that IDE's could not recognise the new methods implemented in \mycompany\CutreApi\ClassOfVendor(). So, for now, I will go with this implementation.
Currently done
namespace mycompany\CutreApi;
use mycompany\CutreApi\ClassOfVendor;
class CutreApi extends \vendor\AwesomeApi\AwesomeApi
{
public function getWhatever(): ClassOfVendor
{
return new ClassOfVendor();
}
}
So, instead of trying to use "whatever" method, I wrote a new one called "getWhatever". In fact both of them are doing the same, just returning a class, but with diferents namespaces as I've described before.
Hope this can help someone.
Related
I was wondering if you could help me with the following question. First of all, I would like to tell you that if I am asking this question here, it is because I have already tried many options and none have worked for me. It turns out that I am developing a package with Laravel and I am using Laravel's dependency injection. But I am at a crossroads from which I have not found a way out. I'm trying to get the instance of a class in an intermediate method from a method chain, let me explain. Here is the code very similar to what I have:
PackageServiceProvider.php
<?php
class PackageServiceProvider extends ServiceProvider
{
public function register()
{
$this->configureBindings();
}
private function configureBindings()
{
$this->app->when(A_Class::class)->needs(B_Interface::class)->give(function () {
return new B_Class();
});
}
...
A_Class.php
<?php
class A_Class implements A_Interface
{
private $b_interface;
public function __construct(B_Interface $b_interface)
{
$this->b_interface = $b_interface;
}
public function create($arg1, $arg2)
{
return $this->b_interface->method_1()->call_another_method_from_another_class();
}
}
A_Interface.php
<?php
interface A_Interface extends Arrayable, Rendereable
{
public function create($arg1, $arg2);
...
}
<?php
class B_Class implements B_Interface
{
public function __construct()
{
// Here is the question...
// How could I get here the instance of the class A_Class?
}
public function method_1()
{
// should return another class instance
}
public function method_2()
{
// should return another class instance
}
}
B_Interface.php
<?php
interface B_Interface
{
public function method_1();
public function method_2();
...
}
If you look at class B_Class``, in the __constructmethod I'm trying to get the instance of classA_Class``` from where that class is being called. I have tried the following:
class B_Class implements B_Interface
{
public function __construct(A_Interface $a_interface)
{
// Here is the question...
// How could I get here the instance of the class A_Class?
}
But I get the following error:
Segmentation fault
I guess there must be some way I can achieve what I need. I would appreciate any help in advance.
Because you are referring to class A inside your class B constructor, and class B in your class A constructor, you have introduced a cyclic dependency.
This will resolve to the error you are experiencing, which is the segmentation fault, as outlined here:
https://laravel.io/forum/11-08-2016-circular-dependency-causes-segmentation-fault-error-when-running-php-artisan-optimize
So the answer is to remove the cyclic dependency if possible, as you can have methods from A calling B that calls A for infinity at runtime, and you will get the above error above again.
If your class A and B are relatively small, I would recommend combining them before using a cyclic dependency.
For interest and prosperity, if you want achieve a cyclic dependency, this is possible by registering your Class A with a singleton from inside A's constructor, and putting the reference to the incomplete object into Class B with your code above. I try with laravels singleton here, its untested, but hopefully you'll get the idea.
class A_Class implements A_Interface
{
public function __construct(B_Interface $b_interface)
{
//I dont think you can pass $this to a function when construction is incomplete, hence $that.
$that = $this;
App::singleton('A_Class', function($that){
return $that;
});
$this->b_interface = $b_interface;
}
}
class B_Class implements B_Interface
{
public function __construct(A_Interface $a_interface)
{
//unfinished object, but no more error.
$this->aClass = App::make('A_Class')
}
}
I would like create an abstract class with abstract method which allow abstract type in return type. In my final class, I would like override type returned with type which implement abstract type initially declared.
<?php
abstract class A {
abstract public function test(A $foo): self;
}
class B extends A {
public function test(B $foo): self
{
return $this;
}
}
This compilation error is thrown:
Fatal error: Declaration of B::test(B $foo): B must be compatible with A::test(A $foo): A in ... on line 8
In documentation, covariance is explained with interface. But not with abstract class. More about PHP implementation, the documentation say:
In PHP 7.2.0, partial contravariance was introduced by removing type restrictions on parameters in a child method. As of PHP 7.4.0, full covariance and contravariance support was added.
I'm using PHP 7.4.
A quite core principle of object oriented programming is the Liskov substitution principle which essentially boils down to:
if S is a subtype of T, then objects of type T in a program may be replaced with objects of type S without altering any of the desirable properties of that program
The way to achieve this is by having covariant method return types, contravariant method type arguments. Exceptions thrown kind of count as return types here so they also need to be covariant.
What you need is covariance of type arguments which breaks this principle.
The reason why can be seen by considering the example below:
abstract class A {
abstract public function test(A $foo): self;
}
class C extends A {
public function test(C $foo): self {
return $this;
}
}
class B extends A {
public function test(B $foo): self {
return $this;
}
}
$b = new B();
$c = new C();
$b->test($c); // Does not work
((A)$b)->test((A)$c); // Works
In the example above, you don't allow B::test to accept any type other than B as a type argument. However since B itself is a child of A and C is also a child of A by simple down-casting (which is allowed) the restriction is bypassed. You could always disable down-casting but that's almost saying you're disabling inheritance which is a core principle of OOP.
Now of course there are compelling reasons to allow covariance of type arguments, which is why some languages (such as e.g. Eiffel) allow it, however this is recognised to be a problem and even has been given the name CATcalling (CAT stands for Changed Availability or Type).
In PHP you can attempt to do runtime checks to remedy this situation:
abstract class A {
abstract public function test(A $foo) {
// static keyword resolve to the current object type at runtime
if (!$foo instanceof static) { throw new Exception(); }
}
}
class C extends A {
public function test(A $foo): self {
parent::test($foo);
return $this;
}
}
class B extends A {
public function test(A $foo): self {
parent::test($foo);
return $this;
}
}
However this is a bit messy and possibly unnecessary.
Strict Standards: Declaration of childClass::customMethod() should be compatible with that of parentClass::customMethod()
What are possible causes of this error in PHP? Where can I find information about what it means to be compatible?
childClass::customMethod() has different arguments, or a different access level (public/private/protected) than parentClass::customMethod().
This message means that there are certain possible method calls which may fail at run-time. Suppose you have
class A { public function foo($a = 1) {;}}
class B extends A { public function foo($a) {;}}
function bar(A $a) {$a->foo();}
The compiler only checks the call $a->foo() against the requirements of A::foo() which requires no parameters. $a may however be an object of class B which requires a parameter and so the call would fail at runtime.
This however can never fail and does not trigger the error
class A { public function foo($a) {;}}
class B extends A { public function foo($a = 1) {;}}
function bar(A $a) {$a->foo();}
So no method may have more required parameters than its parent method.
The same message is also generated when type hints do not match, but in this case PHP is even more restrictive. This gives an error:
class A { public function foo(StdClass $a) {;}}
class B extends A { public function foo($a) {;}}
as does this:
class A { public function foo($a) {;}}
class B extends A { public function foo(StdClass $a) {;}}
That seems more restrictive than it needs to be and I assume is due to internals.
Visibility differences cause a different error, but for the same basic reason. No method can be less visible than its parent method.
if you wanna keep OOP form without turning any error off, you can also:
class A
{
public function foo() {
;
}
}
class B extends A
{
/*instead of :
public function foo($a, $b, $c) {*/
public function foo() {
list($a, $b, $c) = func_get_args();
// ...
}
}
Just to expand on this error in the context of an interface, if you are type hinting your function parameters like so:
interface A
use Bar;
interface A
{
public function foo(Bar $b);
}
Class B
class B implements A
{
public function foo(Bar $b);
}
If you have forgotten to include the use statement on your implementing class (Class B), then you will also get this error even though the method parameters are identical.
I faced this problem while trying to extend an existing class from GitHub. I'm gonna try to explain myself, first writing the class as I though it should be, and then the class as it is now.
What I though
namespace mycompany\CutreApi;
use mycompany\CutreApi\ClassOfVendor;
class CutreApi extends \vendor\AwesomeApi\AwesomeApi
{
public function whatever(): ClassOfVendor
{
return new ClassOfVendor();
}
}
What I've finally done
namespace mycompany\CutreApi;
use \vendor\AwesomeApi\ClassOfVendor;
class CutreApi extends \vendor\AwesomeApi\AwesomeApi
{
public function whatever(): ClassOfVendor
{
return new \mycompany\CutreApi\ClassOfVendor();
}
}
So seems that this errror raises also when you're using a method that return a namespaced class, and you try to return the same class but with other namespace. Fortunately I have found this solution, but I do not fully understand the benefit of this feature in php 7.2, for me it is normal to rewrite existing class methods as you need them, including the redefinition of input parameters and / or even behavior of the method.
One downside of the previous aproach, is that IDE's could not recognise the new methods implemented in \mycompany\CutreApi\ClassOfVendor(). So, for now, I will go with this implementation.
Currently done
namespace mycompany\CutreApi;
use mycompany\CutreApi\ClassOfVendor;
class CutreApi extends \vendor\AwesomeApi\AwesomeApi
{
public function getWhatever(): ClassOfVendor
{
return new ClassOfVendor();
}
}
So, instead of trying to use "whatever" method, I wrote a new one called "getWhatever". In fact both of them are doing the same, just returning a class, but with diferents namespaces as I've described before.
Hope this can help someone.
Is it OK (good practice OOP wise) to make a class implement a certain interface so that a trait being used in that class can access the classes functions.
I found a SO question a few days ago with an answer explaining that traits shouldn't use methods/things from the class it's being used in. What If I made a class implement an interface so it had to have those functions that the trait uses from the class? Would that be OK. I'm taking an OOP class in university next year, so I only learned what OOP I did from the internet, in case this is a bad question. :p
So here's the idea to clarify (in PHP)
trait MyTrait {
public function foo() {
return $this->bar(); // bar is in the class the trait is to be used in
}
}
class MyClass implements MyTraitCompatible {
public function bar() {
return "BAR!";
}
}
interface MyTraitCompatible {
public function bar();
}
Also, is there anyway to enforce that a class needs to implement MyTraitCompatible to use MyTrait?
Edit: (My actual goal is to have one function used in two classes that both extend another class (Eloquent) and would be completely identical but the function would not be used in all classes extending Eloquent - this is one way I thought of doing it.)
One option is that your trait could check that the class using it implements the interface you expect. Here's an example in the constructor method:
trait MyTrait {
public function __construct() {
if (!in_array('MyTraitCompatible', class_implements($this, false))) {
throw new Exception('To use this trait you must implement MyTraitCompatible!');
}
}
public function foo() {
return $this->bar(); // bar is in the class the trait is to be used in
}
}
A valid class would be:
class MyClass implements MyTraitCompatible {
use MyTrait;
public function bar() {
return "BAR!";
}
}
An invalid class would be:
class InvalidClass {
use MyTrait;
public function baz() {
return "I don't think so buddy.";
}
}
Obviously if the class using this trait has a constructor already then this would conflict. There isn't a pretty way to avoid this since the class using the trait will have precedence over the trait and would just override it. One option is that you could define a check method in the trait and call it from the methods in the trait to check compatibility, but it's not ideal:
trait MyTrait {
protected function compatible() {
if (!in_array('MyTraitCompatible', class_implements($this, false))) {
throw new Exception('To use this trait you must implement MyTraitCompatible!');
}
var_dump('Passed the test!');
}
public function foo() {
$this->compatible();
return $this->bar(); // bar is in the class the trait is to be used in
}
}
You could also replace that compatible() method with the __call() magic method - but again, you might run into conflicts if you have one defined elsewhere.
Strict Standards: Declaration of childClass::customMethod() should be compatible with that of parentClass::customMethod()
What are possible causes of this error in PHP? Where can I find information about what it means to be compatible?
childClass::customMethod() has different arguments, or a different access level (public/private/protected) than parentClass::customMethod().
This message means that there are certain possible method calls which may fail at run-time. Suppose you have
class A { public function foo($a = 1) {;}}
class B extends A { public function foo($a) {;}}
function bar(A $a) {$a->foo();}
The compiler only checks the call $a->foo() against the requirements of A::foo() which requires no parameters. $a may however be an object of class B which requires a parameter and so the call would fail at runtime.
This however can never fail and does not trigger the error
class A { public function foo($a) {;}}
class B extends A { public function foo($a = 1) {;}}
function bar(A $a) {$a->foo();}
So no method may have more required parameters than its parent method.
The same message is also generated when type hints do not match, but in this case PHP is even more restrictive. This gives an error:
class A { public function foo(StdClass $a) {;}}
class B extends A { public function foo($a) {;}}
as does this:
class A { public function foo($a) {;}}
class B extends A { public function foo(StdClass $a) {;}}
That seems more restrictive than it needs to be and I assume is due to internals.
Visibility differences cause a different error, but for the same basic reason. No method can be less visible than its parent method.
if you wanna keep OOP form without turning any error off, you can also:
class A
{
public function foo() {
;
}
}
class B extends A
{
/*instead of :
public function foo($a, $b, $c) {*/
public function foo() {
list($a, $b, $c) = func_get_args();
// ...
}
}
Just to expand on this error in the context of an interface, if you are type hinting your function parameters like so:
interface A
use Bar;
interface A
{
public function foo(Bar $b);
}
Class B
class B implements A
{
public function foo(Bar $b);
}
If you have forgotten to include the use statement on your implementing class (Class B), then you will also get this error even though the method parameters are identical.
I faced this problem while trying to extend an existing class from GitHub. I'm gonna try to explain myself, first writing the class as I though it should be, and then the class as it is now.
What I though
namespace mycompany\CutreApi;
use mycompany\CutreApi\ClassOfVendor;
class CutreApi extends \vendor\AwesomeApi\AwesomeApi
{
public function whatever(): ClassOfVendor
{
return new ClassOfVendor();
}
}
What I've finally done
namespace mycompany\CutreApi;
use \vendor\AwesomeApi\ClassOfVendor;
class CutreApi extends \vendor\AwesomeApi\AwesomeApi
{
public function whatever(): ClassOfVendor
{
return new \mycompany\CutreApi\ClassOfVendor();
}
}
So seems that this errror raises also when you're using a method that return a namespaced class, and you try to return the same class but with other namespace. Fortunately I have found this solution, but I do not fully understand the benefit of this feature in php 7.2, for me it is normal to rewrite existing class methods as you need them, including the redefinition of input parameters and / or even behavior of the method.
One downside of the previous aproach, is that IDE's could not recognise the new methods implemented in \mycompany\CutreApi\ClassOfVendor(). So, for now, I will go with this implementation.
Currently done
namespace mycompany\CutreApi;
use mycompany\CutreApi\ClassOfVendor;
class CutreApi extends \vendor\AwesomeApi\AwesomeApi
{
public function getWhatever(): ClassOfVendor
{
return new ClassOfVendor();
}
}
So, instead of trying to use "whatever" method, I wrote a new one called "getWhatever". In fact both of them are doing the same, just returning a class, but with diferents namespaces as I've described before.
Hope this can help someone.